1. Technical Field
The present invention relates to a rubber composition, and more specifically, relates to a rubber composition having an improved dispersibility of an inorganic filler, as well as improved low heat generation property and tensile stress.
2. Description of the Related Art
For some of the recent pneumatic tires, a silica is blended as a reinforcing filler with a tread rubber in place of part or all of carbon black in order to improve the fuel efficiency. Blending the silica reduces a hysteresis loss (tan δ) of a pneumatic tire, and thus reduces a rolling resistance (heat generation) of the tire. However, a poor dispersibility of the silica in a rubber causes the following problems. Specifically, it is difficult to achieve a low heat generation property even by blending the silica, and also, it is impossible to obtain a sufficient rubber strength (particularly, a tensile stress at 1000 strain) of the rubber composition when the silica is blended. Against these problems, it has been known that a sulfur-containing silane coupling agent is blended with a rubber composition in addition to the silica so as to improve the dispersibility of the silica (refer to, for example, Japanese Patent Application Kokai Publication No. Hei 7-48476).
However, since blending a sulfur-containing silane coupling agent makes a premature cure (scorching) more likely to occur, the amount of the sulfur-containing silane coupling agent to be blended is limited. This limitation makes it difficult to further improve the dispersibility of the silica. As a result, the sulfur-containing silane coupling agent has a limited ability to further improve the low heat generation property and the tensile strength of a rubber composition. While some inorganic fillers other than the silica also have poor dispersibilities in a rubber, the use of the sulfur-containing silane coupling agent has a limited effectiveness in improving the dispersibilities of such inorganic fillers.